1. Field of the Invention
The present invention relates generally to improvements in fringe-field planar-magnetic speakers; and, more particularly, to fringe-field planar-magnetic speakers with single-ended primary magnetic circuits.
2. Background
Two general fields of loudspeaker design comprise (i) dynamic, cone devices and (II) electrostatic thin-film devices. A third, heretofore less exploited area of acoustic reproduction technology is that of thin-film, fringe-field, planar-magnetic speakers. This third area represents a bridging technology between these two previously recognized general areas of speaker design; combining a magnetic motor of an electro dynamic/cone transducer with a film-type diaphragm of a electrostatic device. However, it has not produced conventional planar-magnetic transducers, which, as a group, have achieved a significant level of market acceptance over the past 40-plus years of evolution. Indeed, planar-magnetic speakers currently comprise well under 1% of the total loudspeaker market. It is a field of acoustic technology that has remained exploratory, and embodied only in a limited number of relatively high-priced commercial products over this time period.
As with market acceptance of any speaker, competitive issues are usually controlling. In addition to providing performance and quality, a truly competitive speaker must be reasonable in price, practical in size and weight, and must be robust and reliable. Assuming that two different speakers provide comparable audio output, the deciding factors in realizing a successful market penetration will usually include price, convenience, and aesthetic appearance. Price is primarily a function of market factors, such as cost of materials and assembly, perceived desirability from the consumer's standpoint (as distinguished from actual quality and performance), demand for the product, and supply of the product. Convenience embodies considerations of adaptation of the product for how the speaker will be used, such as mobility, weight, size, and suitability for a customer-desired location of use. Finally, the aesthetic aspects of the speaker will also be of consumer interest; including considerations of appeal of the design, compatibility with decor, size, and simply its appearance in relation to the surroundings at the point of sale and at the location of use. If planar-magnetic speakers can be advanced so as to compare favorably with conventional electro dynamic and electrostatic speakers in these areas of consideration, further market penetration can be possible; as reasonable consumers should adopt the product that provides the most value for the purchase price paid.
With this background, a discussion of the relative successes and failures of conventional planar-magnetic speakers, and design goals and desired traits of operation will be given. It is interesting to note that the category of fringe-field, planar-magnetic speakers has evolved around two basic categories: a)“single-ended”; and, b) symmetrical “double-ended” designs, the later sometimes being called “push-pull,” and both will be touched on as background for discussion of single-ended designs.
A conventional push-pull device is illustrated in FIG. 1. This structure is characterized by two magnetic arrays, 10 and 11, each supported by perforate substrates 14, 24; and positioned on opposite sides of a flexible diaphragm 12 which includes a conductive coil 13. The film is tensioned into a planar configuration. An audio signal is supplied to the coil 13, and a variable voltage thereby provided in the coil interacts with the otherwise fixed magnetic field between the magnet arrays 10 and 11. The diaphragm is displaced in accordance with variations in the audio signal, thereby generating a desired acoustic output. A representative example can be found in the disclosure of U.S. Pat. No. 4,156,801 issued to Whelk.
Because of a doubled-up, front/back magnet layout of prior push-pull planar-magnetic transducer structures, these double-ended systems have been generally regarded as more efficient, but as more complex to build. Also, they have certain performance limitations stemming from the formation of cavity resonances derived from the passage of sound waves through the cavities, or channels 16 formed by the spaces between the magnets of the arrays 10, 11, and acoustically radiating to the external environment through holes 15 in the substrates 14, 24. This can cause problems at certain frequencies, including giving rise to resonant peaks and band-limiting attenuation. In all fairness it must be said that single-ended designs are not immune from this problem; and particularly where the magnet spacing is close together, cavity resonances can occur in single-ended as well as double-ended designs.
Double-ended designs are also particularly sensitive to deformation from repulsive magnetic forces, which tend to deform the structures of such devices outward. This outward bowing draws the edges of the diaphragm closer together and alters the tension on the diaphragm. This can significantly degrade performance, to the point of rendering the speaker unusable.
As mentioned, a second category of planar-magnetic speakers comprises single-ended devices. With reference to FIG. 2, a typical conventional single-ended speaker configuration, having a flexible diaphragm 17 with a number of conductive elements 18, is set forth by way of example. The diaphragm is tensioned and supported by frame members (not shown) carried by a substrate 19 of the frame, and which frame members extend outward (upward in the figure) beyond the top of a single array of magnets 35 to position the diaphragm an offset distance away from the tops of the magnets to accommodate vibration of the diaphragm. The array provides a fixed magnetic field with respect to the coil conductors 18 disposed on the diaphragm. It will be apparent that the single array of magnets (typically of ceramic or rubberized ferrite composition) provides a much-reduced energy field, compared to the previously discussed push-pull device, assuming comparable magnets are used. Because of this and other reasons, previous single-ended devices of compact size have not provided performance that has been deemed acceptable for commercial applications.
Furthermore, conventional single-ended devices have had to be quite large to work effectively; and, even so, are less efficient than standard electrostatic and electro dynamic loudspeaker designs mentioned above. Small, or even average-sized single-ended planar-magnetic devices (compared to electro dynamic and electrostatic speakers) have not effectively participated in the loudspeaker market in the time since the introduction of planar magnetic speakers. Comparatively large devices, generally greater than 300 square inches, have been available to consumers in the speaker market; and these exhibit limited competitiveness. That is to say, they are on par with standard speakers in terms of acceptance, suitability to certain applications, cost, and performance. But again, the market penetration of planar-magnetic speakers is less than 1%, including both single-ended and push-pull devices. Prior single-ended planar-magnetic devices with such large diaphragm areas require correspondingly relatively large and expensive structures; and, such relatively larger speakers can be cumbersome to place in some environments. They have relatively low efficiencies as well, compared with electro dynamic and electrostatic speakers, requiring more powerful, and hence more expensive, amplifiers to provide adequate signal power to drive them.
At first impression, a single-ended device might appear to be simpler and cheaper to build than a double-ended design. The same amount of magnet material can be used by doubling the thickness of the magnets to correspond to the combined thickness of a double-ended array of magnets. Because magnets of a given material made twice as thick are cheaper installed than twice as many magnets half as thick (as in a double-ended device) there should be significant savings in a single-ended configuration. Furthermore, the structural complexity is significantly less with regard to single-ended designs, further adding to expected cost savings.
However, doubling the depth of the magnets from that of most designs does not achieve the expected design goal of providing twice the magnetic energy in the gap between the diaphragm and the array of magnets when using conventional ferrite magnets. Accordingly, the expectation for lower cost per a given performance level in the single-ended device has not been realized. Some attempts to improve the design of single-ended planar-magnetic devices have involved the use of relatively many more, and very closely spaced, magnets to provide sufficiently high magnetic field energy. Even then, however, the planar area must be very large, using even more magnets to generate enough sensitivity and acoustic output. For at least these reasons, prior attempts to develop a commercially acceptable single-ended planar-magnetic device have not achieved the desired lower cost for comparable performance design goals. This is true even though the basic form of their structure would seem to be simpler than push-pull devices. And again, the design has not obviated the need for a large surface area and therefore a large device compared with most other speaker types.
Moreover, the architecture of the double-ended planar-magnetic loudspeaker is quite different from that of a single-ended design. For example, the magnetic circuits of the front and back magnetic structures interact, and require a different set of design parameters, e.g. Spacing, field energy, and spatial relationships between the essential elements, to be optimized for best results. Very few of those interactive relationships are transferable in relation to design of single-ended transducers, which have their own unique set of optimal relationships between the essential elements involved.
As mentioned, prior planar-magnetic speakers, particularly prior art single-ended devices, have utilized rows of magnets placed closely, side-by-side to provide improved performance. The magnets are oriented so as to have alternating polarities facing the film diaphragm 17, which carries conductive wires or strips 18 placed conventionally so as to be substantially centered between adjacent magnets. Such prior devices further illustrate that the magnetic field energy to be interacted with by the variable fields set up by the variably energized conductive strips is a shared magnetic field with lines of force arcing between adjacent magnets. In such prior devices, the available magnetic force to be exploited is assumed to be at a maximum at a point half way between two adjacent magnets of opposite polarity orientation; and correspondingly, centered placement of the conductive strips in the field at that location is typical. To achieve sufficient flux density at the position centered between the magnets, it has been shown that (i) not only does the total size of the system need to be increased; but, (II) the magnet placement must be much closer together and more plentiful in a single-ended device than in a push-pull planar-magnetic transducer.
Further, in contrast with standard, dynamic cone-type speakers, thin film planar loudspeakers have a critical parameter that must be optimized for proper functionality. The parameter is film diaphragm tension (See, for example, U.S. Pat. No. 4,803,733 to Carver). Proper, consistent, and long-term stable tensioning of the diaphragm in a planar device is very important to the performance of the loudspeaker. This has been a problematic area of consideration for thin-film planar devices for many years, and it is a problem in design and manufacture for current thin-film devices. Even the most carefully adjusted device can meet short-term requirements, but still can still have long-term problems with tension changes due, for example, to the dimensional instability of the diaphragm material and/or diaphragm mounting structure. Compounding this problem is force interaction within the magnet array and the supporting structure. Due to close magnet spacing of single-ended magnetic structures, the magnetic forces of the adjacent rows of magnets can interact and attract/repel each other to a greater or lesser degree depending upon the polarity relationship of the magnets and their spacing. The interaction over time can cause materials to deform; and impose changes on the film tension. This can degrade the performance of the speakers over time.
Electrostatic loudspeakers have critical diaphragm tension issues, but they do not have magnetic forces working to change the tension in the same way or to the same degree. Dynamic cone-type speakers have magnetic coil transducers and strong related forces, but do not utilize tensioned diaphragms. Planar-magnetic speakers, and particularly single-ended configurations, pose unique challenges with respect to long-term stability for diaphragm tensioning.
With conventional planar-magnetic speakers an increase in magnetic energy derived by increasing the number, or the strength, or both, of the magnets in the magnetic structure further exacerbates the problem of magnetic forces interference with calibrated film tension. Per the foregoing, this is true particularly over time. These and other problems are known to many practitioners in the art. Another example of a prior art single-sided planar-magnetic device, which further illustrates some of these issues, is set forth in U.S. Pat. No. 3,919,499 to Winey.
Turning now to more particular consideration of the magnets themselves, the selection of proper magnets for planar-magnetic speakers is an important consideration. High-energy neodymium magnets have been available for over ten years, and have been used in electro dynamic cone-type speakers. As will be appreciated, however, such speakers do not employ magnetic material structures and supporting structures to support the magnets and at the same time maintain a tension on a nominally flat diaphragm that can be influenced by the magnets. Such relatively more high-energy neodymium magnets have not been effectively applied to single-ended planar-magnetic transducers over this past decade wherein they have been widely available. This is true even though there has been a great need for an improved magnetic circuit to enhance speaker output and reduce size.
One possible explanation for this is that practitioners in planar-magnetic speaker technologies already have difficulty with the critical aspect of diaphragm tensioning. As mentioned, not only is it necessary to achieve a proper initial diaphragm position and tension, but that this configuration must be maintained over years of use, despite inter-magnetic forces, tension forces, and stress arising during dynamic vibration of the diaphragm, all of which can deform supporting and stabilizing structure materials. These factors affect dimensional stability of such structure, as they are constantly working over time to change the magnet positioning and structural frame shapes, such that the diaphragm tension and a magnet-to-diaphragm distance can be influenced. Over a relatively short or long period of time, this tends to un-calibrate the diaphragm tension and degrade the performance of the speaker. It only takes a change of a fraction of a millimeter to significantly alter the performance of a thin-film planar-magnetic loudspeaker. Since this problem is already pivotal in the performance and lifetime reliability of planar-magnetic transducers, exacerbating the problem further with use of magnets having 5 to 40 times the interactive forces would not appear likely to function reliably as a substitution for conventional magnets which already destabilize in the lower-energy magnetic fields used in single-ended planar-magnetic loudspeakers in the current state of the art.
With current magnetic structure designs of single-ended planar-magnetic loudspeakers having the very close side-to-side spacing, as compared to double-ended designs mentioned above, a perceived problem with high-energy magnets is that the attractive forces of the magnets would appear to be too intense; to a point of not only potentially distorting the structure, and affecting diaphragm tension, but even affecting the stability of existing magnet attachment means. For at least these potential reasons, such high-strength magnets have not been successfully used in a commercial planar-magnetic design.
Another difficulty with conventional single-ended planar magnet loudspeaker designs is that of low-frequency range distortion. Since most commercial planar-magnetic speakers do not provide the extended low-frequency performance of a dedicated sub woofer, there has been a need for integrating the planar loudspeaker with a sub woofer in an audibly seamlessly fashion. Due to relatively poor damping of prior-art planar-magnetic loudspeakers, more particularly single-ended ones, there have been high “Q” resonances at the low frequency end of the planar-magnetic system response range, which is at or near the transition frequency to a sub woofer. Because of this discontinuity, the audible result is often poor, with clearly detectable adverse coloration of the sound due to this problem. For at least this reason, there is a need for improved damping at the fundamental resonant frequency of single-ended planar-magnetic speakers to lower distortion.
Further, combination of thin-film diaphragms and conductive materials of the attached coil of prior planar-magnetic speakers has presented design challenges. Polyester diaphragms that have often been used in prior planar-magnetic transducers have exhibited poor thermal stability and poor dimensional stability at elevated temperature. This has heretofore been a practical limitation to increased sound pressure levels with single-ended planar-magnetic systems due to thermal instability limitations of the diaphragms; and, also, of de-bonding of adhesives used to attach conductive wires and/or strip regions to such diaphragms. Thermally-induced deformation problems have been further magnified by low efficiency due to relatively poor magnetic coupling in prior single-ended devices, requiring greater power input to the conductive coil, more localized heating, and therefore requiring greater thermal dissipation for a given acoustic output level. Accordingly, there is a need for a diaphragm/conductive coil combination with greater thermal and dimensional stability to maintain proper tension.
In summary, heretofore neither double-ended or single-ended designs of planar-magnetic loudspeakers have reached a stage of development which enables them to be favorably competitive with speakers of the first two types discussed above (dynamic and electrostatic) having much less stringent manufacturing requirements, smaller size, higher efficiencies, and lower costs. This lack of market success has continued over a period of more than 40 years since planar-magnetic acoustic transducers were first disclosed. As mentioned, even the appearance, over the last decade, of high energy magnets such as those comprising neodymium have heretofore not been exploited to offer needed improvements, particularly within single-ended speaker structures.